Alzheimer's disease (AD) is a devastating disease of the aged. Two amyloid-forming proteins are associated with AD, the amyloid ?-protein (A?) and tau. Recent evidence supports an hypothesis, the amyloid cascade hypothesis, that posits that A? oligomers are the seminal neuropathogenetic agents in AD. The overall goal of this proposal is to understand the structural biology of A?, and fragments thereof, and to establish formal structure-activity relationships. In the long run, we seek to obtain an atomic-resolution determination of the structure of the proximate neurotoxins formed by A?, and in doing so, enable the development, for the first time, of disease-modifying AD treatments. A multidisciplinary strategy, employing complementary experimental and computational approaches, will be employed. This strategy has been used very successfully in the past, providing novel insights into the A? system. Three specific aims are proposed that systematically and logically progress from in vitro biophysical studies of A? and its oligomeric assemblies (Aim 1), to in vitro and in vivo studies of the biological activity of selected such assemblies (Aim 2), to determination of the effects of selected assemblies on differential gene expression in neurons (Aim 3). Taken together, these studies will provide the theoretical and experimental foundation for subsequent therapeutic compound development and clinical testing in humans. Aim 1. To determine the structural dynamics of A? and tau assembly. a. To use scanning Tyr mutagenesis to elucidate mechanisms of A? oligomerization. b. To determine the dynamics of intramolecular turn formation and its effects on A? assembly. c. To determine the effects of primary structure changes on the conformations and assembly dynamics of biologically relevant and theoretically important A? peptides. Aim 2. To determine the biological effects of A? assemblies. a. To determine the cytotoxic effects of A? assemblies on cultured neuronal cell lines and primary neurons. b. To determine the effects of A? assemblies on Drosophila eye development, locomotion, and longevity. Aim 3. To identify and validate AD-relevant genes using a bioinformatics approach that considers A? assembly structure, neuron type, and neuron senescence.